A significant amount of water is wasted at some hydroelectric dams through leakage of water through poorly sealed sluice gates or stop logs. If diverted through the turbines, this wasted water flow could generate significant additional kilowatts of electricity. Wastage through water leakage is a particular concern during low water seasons or periods of peak power demand.
Many years ago, it was discovered that coal cinders would stop leaks in hydro dams. The stop logs in sluice gates were traditionally made from wood, although steel and other materials may now be used. The process of using coal cinders for stopping leaks was referred to as “cindering”. Cinders would simply be dropped into the water of the head pond near the leak, and as the cinders would sink, the cinder particles would be drawn into the leak by the current, and collect to seal the leak. This solution was effective and avoided the need for divers to go down to patch the leak. Coal cinders were abundant and inexpensive. Coal cinders for use as a dam sealant were stripped from abandoned rail beds, from boilers of steam locomotives, or coal ash was purchased from institutions that used coal for fuel, for example.
Leaks may also occur around head gates, wicket gates, cracks in the concrete structure and, on some occasions, the bedrock below and around the dam. Coal cinders are also effective for these applications. In some cases, coal cinders may be used as a temporary fix. They are also important for use as sealants during maintenance operations.
Wicket gates are used to control the amount of water entering the turbine from the penstock. The penstock brings the water down from the head pond through the head gate entrance. Hydroelectric dam operators have a number of different configurations in their powerhouses. In one example, one operator may have 10 units, i.e., generators with their own individual headgates, penstocks, wicket gates, turbine, draft tube and tail race gates. They all do preventative maintenance programs, where they will take one unit out of service to do overhaul from top to bottom. This will include the turbine, which is usually under water rotating at high speed. Preventative maintenance involves a procedure called de-watering the generator or turbine. To do this they must stop the flow of water coming in. Up top, they drop a metal gate called a head gate. This stops the water from flowing into the intake and down the penstock. Since seals are not perfect, these gates always tend to leak around the edges. To de-water the turbine, sealant material is needed to seal leaks around the head gate to stop the water flow completely. Maintenance personnel can safely enter the water turbine area (which is usually submerged when the turbine is running during operation) only after the water flow is stopped. Imagine being down in that dark place in the turbine area, looking up through the wicket gates, up through the penstock and thinking about all that water that could come rushing down if the sealant failed. Thus, maintenance personnel need to have confidence in the reliability of the sealant.
Usually dam operators acquire experience with, and confidence in, the sealant material when doing routine maintenance on the sluice gates and sealing the stop logs. Then, when the time comes to do the critical maintenance involving sealing the head gate for de-watering the turbine, they have confidence in the reliability of the sealant.
To be more effective, a particular mix and specific gravity is required of the aggregate for sealing dams, i.e., the coal cinders are required to have the right gradation, ranging from large pieces down to fine sand sized particles. Depending on what the dam operator is trying to accomplish, a 2″ minus mixture or a 1½″ minus mixture of cinders, for example, may be optimum. A coarser 3″ minus aggregate may be needed if there are larger holes, for example. Specially graded coal cinders for use as leak sealants for dams have been produced by J. M. Power Aggregates Limited.
More recently, concerns have been raised about the heavy metal content of coal cinders and the environmental impact of contaminants in coal cinders. In the 1990s demand for coal cinders for this purpose dropped off. Ontario Hydro, for example, sought to find more environmentally friendly materials. In 1985, in Canada, the Department of Fisheries banned the dumping of coal ash, stones and deleterious substances into water where fish live. Thus, even if the use of coal cinders is still permitted, strict handling procedures are required to ensure coal cinders meet environmental requirements.
Another known alternative to coal cinders for this purpose is an aggregate made from blast furnace slag, marketed under a trade name of Hydro-Lite™, by Lafarge Canada Inc. This material is used just like coal cinders as a leak sealant. However, some dam operators have reported that the slag-based product is not as effective or reliable as coal cinders. They expressed a lack of confidence in it compared with the traditional coal cinders, particularly during critical maintenance involving de-watering the turbine. Also, the composition of blast furnace slag tends to vary significantly from batch to batch, since it is a by-product from smelting ores from different sources. Thus, each batch of slag must be tested and pass leachate tests for potential environmental contaminants.
Conventional heavy aggregates such as crushed stone and gravel have been tested and found to be ineffective or unreliable as leak sealants for this purpose. Polished sand and gravel tend to wash through the leak. In looking at alternative materials, it was also discovered that there has to be a certain amount of abrasive edges on the material to mechanically lock in the holes in the dams. However, if the pieces of material are too large or rough, they may interfere with locking mechanisms of head gates, tail race gates and around the gains of sluice gates. It is also desirable to avoid materials that are damaging to the environment, e.g. have unacceptable levels of contaminants and/or are harmful to aquatic or marine wildlife.
Thus, it would be desirable to find an alternative material or method that avoids or reduces the above-mentioned problems, and preferably one that is environmentally friendly, to replace known solutions such as coal cinders or blast furnace slag based aggregates for use as leak sealants for dams and other water retaining structures.